1. Field of the Invention
The present invention relates to cam operated cylindrical clamps for securing heaters onto injection molding nozzles.
2. Description of Prior Art
Injection molding machines typically have a cylindrical nozzle which engages with a receptacle on a mold. Thermoplastic material is injected through the nozzle into the mold. A cylindrically wound electric heater element is used to maintain proper nozzle temperature. A cylindrical clamp which surrounds the heater element is used to maintain the appropriate thermal contact between the heater and the nozzle. Proper thermal contact is necessary to maintain the quality of the molding operation and to maximize the life of the heater element. Depending on the configuration of the molding equipment, the heater clamp on the injection nozzle is often difficult to access with adjustment tools and visual access also can be limited.
Prior art clamps use various mechanisms to provide the clamping action. Two such clamps using different mechanisms are shown in U.S. Pat. Nos. 4,968,247 and 4,859,176.
The clamp as shown in U.S. Pat. No. 4,859,176 to Meyer has 5 moving parts. The tightening of the clamp is accomplished by rotating 2 screws. Typically, the adjustment tool such as an Allen wrench will have to be inserted into each of the adjustment screw heads and rotated through an arc repeatedly until the clamp is adequately tightened. The user subjectively determines by feel when each screw and the clamp are sufficiently tight. The clamp tightness is maintained by internal friction of the device. Depending on several variables, including the physical and visual accessibility of the heater clamp in the molding equipment, the tightening process can take several minutes.
The clamp shown in U.S. Pat. No. 4,968,247 to Olson shows a configuration of a cam operated heater clamp. The clamp utilizes a cam pin and cam follower loop as the clamping mechanism where the tightness of the clamp is adjusted by rotation of the cam pin. The rotational position of the cam pin for maximum clamping effect is where the cam portion is directly opposite the connection point between the cam follower loop and the clamp housing with reference to the central axis of the cam pin hubs. At this rotational position the circumference of the circular clamp housing is at its minimum, the clamping force is at the maximum, and the tension in the clamp housing and cam follower loop is at the maximum. Internal friction, primarily between the cam pin and the cam journals, and between the cam portion and the cam follower, creates the rotational resistance of the cam pin. As the tension in the cam follower loop increases the internal friction increases and consequently the rotational resistance of the cam pin increases. At the point of maximum clamping effect the rotational resistance is at the maximum. The rotational resistance is relied upon to hold the clamp in its clamped position.
The rotational position of the cam pin for maximum clamping effect during adjustment of the Olson clamp can be determined by one of two means. First, by rotating the cam pin back and forth through an arc while subjectively feeling the rotational resistance of the cam pin and estimating the approximate position of maximum tension in the clamp. Secondly, by visually observing the position of the cam pin while adjusting it.
Either method of determining the proper rotational position during adjustment is subject to error, especially if visual or physical access to the nozzle is limited, such as by the molding assembly. When the adjustment of the cam pin does not set the cam portion at the proper rotational position for maximum clamping effect, the tension in the cam follower loop provides a component of force that effects a torque on the cam pin which may inadvertently loosen the clamp. This is especially the case where there is not adequate internal friction in the cam parts or where there are vibrational forces present. When the clamp loosens, proper thermal contact between the heater element and the nozzle is lost which can cause problems in the molding operation and a shortened life expectancy of the heater element.
It is desired to have a quicker, simpler, and more definitive means of determining the appropriate position of the cam pin during adjustment for optimal tightness and a reliable locking means to prevent inadvertent loosening of the clamp.